1. Field of the Invention
The present invention relates to a process for preparing fluids for use in petroleum recovery, to fluid compositions for use in petroleum recovery, and to methods for using such fluids in petroleum recovery.
2. Brief Description of the Prior Art
Petroleum recovery from underground formations can be a multistage process. While some petroleum may occur under pressure in such formations and may be extractable using primary processes depending on the pressure of the petroleum in the formation to expel the petroleum from the formation, petroleum often remains after primary processes have become ineffective. In such cases, secondary and tertiary processes may be useful to extract additional petroleum from the formations. In these latter processes, a fluid is typically injected into the formation under pressure through a first well to flood or drive residual petroleum to one or more extraction wells. The flooding fluid can be a gas such as carbon dioxide, or a liquid, such as water obtained locally, such as fresh or sea water. Depending on the nature of the formation, it is often desirable to increase the viscosity of the flooding fluid. This can be accomplished by the addition of a polymeric material to the flooding fluid. The polymeric material may be a natural polymeric material of biological origin, such as, for example, biopolysaccharides, disclosed in U.S. Pat. No. 4,458,753, incorporated herein by reference, or a synthetic polymer, such as, for example, polyacrylamide.
Dissolved oxygen in the flooding fluid can present a number of problems. For example, in the case of biological polymers in particular, dissolved oxygen can accelerate the degradation of the polymer, resulting in an undesirable reduction in the viscosity of the flooding fluid. In addition, dissolved oxygen can have a corrosive effect on equipment used to provide the flooding fluid to the formation, such as pumps, mixing equipment, well casings and the like.
Numerous attempts have been made to reduce the amount of dissolved oxygen in such flooding fluids. For example, U.S. Pat. No. 4,414,334 discloses the use of alcohol to react with dissolved oxygen in the presence of an alcohol oxidase to remove the free oxygen from a fluid. Sodium dithionite compositions have also been employed, such as disclosed in U.S. Patent Publication No. 2011-0232904 A1, incorporated herein by reference. U.S. Pat. No. 4,059,533, incorporated herein by reference, discloses mixtures of sodium dithionite and sodium diothionate as oxygen scavengers in petroleum drilling fluids and in secondary and tertiary recovery operations.
Biopolymers such as xanthan gums, produced by fermentation of using bacteria of the genus Xanthomonas, and similar biopolysaccharides, produced by fermentation of bacteria of the genus Sclerotium and of the genus Alcaligenes, have been used in flooding fluids, such as disclosed in U.S. Pat. No. 3,305,016, U.S. Pat. No. 3,372,749, and British Patent Specification 2,082,189, respectively. U.S. Pat. No. 4,458,753, incorporated herein by reference, discloses the addition of an alkali metal borohydride to an aqueous solution of such biopolysaccharides improves the high temperature stability of the biopolymer. According to U.S. Pat. No. 4,218,327, incorporated herein by reference, the viscosity of xanthan gum-containing flooding fluids can be stabilized by removing substantially all dissolved oxygen, and then adding the xanthan gum polymer, at least one sulfur-containing antioxidant, and at least one easily oxidized water-soluble alcohol or glycol.
When ferric chloride plugging of water-flood flooding wells has been encountered, the addition of a water-soluble hydrosulfite, such as potassium, sodium, or ammonium hydrosulfite has proven to be useful in reducing oxidation of soluble ferrous chloride to insoluble ferric chloride, according to U.S. Pat. No. 3,343,601, incorporated herein be reference. U.S. Pat. No. 3,399,725, incorporated herein by reference, discloses a process for recovering crude oil from subterranean oil-bearing formations, using an aqueous flooding medium substantially free of molecular oxygen, and containing a water-soluble, substantially linear organic polymer. Ferrous ions are encountered in most hydrocarbon-bearing formations, and oxidation to the ferric state can result in degradation of the organic polymer, reducing the efficacy of the recovery process.
When polymeric materials are included in aqueous fluids for use in petroleum recovery, the polymeric material must be dissolved or dispersed in the fluids, typically by mixing operations. Such mixing operations create turbulence, which can simultaneously substantially increase the dissolved oxygen level in the aqueous fluid, to the detriment of subsequent petroleum recovery operations.
There is a continuing need to reduce the dissolved oxygen level of aqueous fluids employed in petroleum recovery operations.